In producing chemical pulp according to the Kraft chemical pulp process, waste liquor is produced that is being evaporated prior to burning. During the evaporation process, liquor vapor is stripped off, which in addition to water vapor, also contains certain volatile contaminants. Such contaminants are hydrogen sulfide, methylmercaptan, dimethylsulfide, methanol, terpenes etc. At the evaporation which takes place as a so called multiple effect evaporation with a number of stages, effects (normally 4-7), the liquor vapor is also condensed in multiple stages, whereby also large amounts of the volatile contaminants will condense. The condensation takes place in at least as many stages there are effects. This means that the quality of the condensate varies significantly from the different stages of the evaporation. Normally 2-3 different condensate qualities are being separated, where each one is a mixture of condensates from a number of effects. The dirtiest condensate, (foul condensate), is normally treated in a steam stripper where the volatile components are flashed off. This foul condensate is typically a small amount of the total condensate flow and therefore the steam economy is not affected to any higher degree of the fact that steam is used as the stripper gas. The investment cost can also be kept at a minimum.
The purity of the other condensate qualities is highly dependent on the amount of foul condensate. If the amount of foul condensate is increased the contaminated condensates will be cleaner. A too high amount of foul condensate however the operating and investment cost for the steam stripper system will increase.
The other, less contaminated condensates can to a limited extent be used as process water in dependency of their cleanliness. However if the condensate is too contaminated it can not be re-used but must instead be discharged to the recipient subsequent to some form of treatment
The primary limiting factor for the use of the contaminated condensate as process water is the content of sulfides, as these can give an unpleasant small and taste to the pulp. It also creates a significant problem for the working environment. Also terpenes give a smell. The terpenes however are normally present at very low amounts in the less contaminated condensates.
The technology available to clean these condensates is predominately steam stripping. Since the various condensate flows are very large, the size of the stripper will be significant and a large amount of steam will be required for stripping. The steam volumes will be so large that it will definitely not be economical to use fresh steam. On the other hand it is possible to use flash steam driven off from the evaporation of the waste liquor, in multiple effect evaporation for the stripping. The steam leaving the stripper then can be regained as heat in the next evaporation effect. The cleaning efficiency of such a stripper is however limited since the flash steam from the preceding effect is already contaminated with sulfides, which limits the degree of purity of the output condensate. Primarily the cleanliness is limited regarding sulphides, as the waste liquor can have a considerable content of sulphides. This sulphide content is dependent on that steam is normally taken from the first effect, where the temperature is rather high, which gives an increased sulphide content.
Another drawback is that when the steam passes through the stripper, it loses pressure and volatile components are enriched. These two things will reduce the condensation temperature, which means that the temperature difference available at the evaporation is reduced. The energy and capital cost are both negatively impacted thereby. Furthermore the evaporation plant and the stripper are completely integrated, whereby these two parts can not be independently operated.
The dimensions of the stripper also will become large, which means significant costs for the equipment.
In a conventional steam stripper also other volatile components, such as methanol, are stripped off.
Air can be used to in lieu of steam to strip the condensates. A big drawback with this method is that air is being contaminated and must be cleaned in some way. The air volumes can also be very large. Additionally the condensate is being cooled down by the air, which has a lower wet bulb temperature as compared to the temperature of the condensate. For these reasons pure air stripping is not a realistic alternative for a modern and environmentally friendly pulp mill.
The present invention provides a possibility to strip off primarily sulfides at a very high efficiency from liquor-steam condensates from a pulp manufacturing process, and simultaneously to take care of the sulphur, thus that it will not contaminate the environment. This is being done in a closed loop concept that is comprised of three process steps, where the sulfides are stripped off from the condensate, the stripped off sulfides are being oxidized to sulphur dioxide, and to absorb the sulphur dioxide formed.
The three process steps are consequently:
1. Stripping off sulphides from liquor-steam condensate
2. Oxidation of combustible components such as sulphides and hydro carbons.
3. Absorption of sulphur dioxide.
By integrating these three process steps (1, 2, and 3) in a closed loop cycle, the cleaning of condensates can be done with a high efficiency, good heat economy, and minimal impact on the environment